1. Field of the Invention
This invention relates to organic compounds and, in particular, organic compounds exhibiting photoelectronic behavior, as well as processes based on such behavior.
2. Art Background
Recently there has been significant interest in organic materials that are either conductive or that can be doped to produce conductivity. Such organic materials have been suggested for a wide variety of uses that depend on their conductivity. For example, organic materials are generally easily formed in thin films as conductive components in devices such as switches.
The classic carbon-based conductors are graphite and polyacetylene. (Graphite is characterized by an infinite sheet-like structure of the element carbon.) Graphitic materials typically have conductivities in the range 10.sup.3 to 10.sup.5 Siemens/cm, but are intractable and therefore for some applications do not lend themselves to fabrication of the desired devices. Polyacetylene when doped has conductivities as high as 10.sup.4 Siemens/cm, but can be processed only during its preparation. Other organic conductors, such as those based on tetrathiafulvalene, have high conductivities (10.sup.3 Siemens/cm), but again are difficult to form into desired geometries. (See, for example, U.S. Pat. No. 4,249,013 dated Feb. 3, 1981.)
A class of nascent carbon based materials are those based on fullerenes. Such materials are prepared by an electronic discharge process as described in H. W. Kroto, et al., Nature 318, 162 (1985) and W. Kratschmer, et al., Nature 347, 354 (1990). These compositions, as reported, are wide band gap materials. Attempts have been made to modify these materials to improve their conductivity. For example, as reported by F. Wudl, at the Materials Research Society Meeting, Nov. 29, 1990, Boston, MA., a tetraphenylphosphonium salt of fullerene has been made, but exhibited a conductivity no greater than 10.sup.-5 Siemens/cm.
A variety of organic materials have also been found to exhibit various electronic effects induced by their interaction with electromagnetic radiation. For example, phthalocyanine derivatives have been shown in a liquid junction solar cell to exhibit a photovoltaic effect, and thus to allow generation of an electrical current when the organic material is illuminated with radiation in the wavelength range 400 nm to 900 nm. (See D. Guay et al, Journal of the Electrochemical Society, 134, 2942 (1987).) Additionally, materials such as anthracene, have also exhibited photoconductivity as described by Gutmann and Lyons in Organic Semiconductors, Wiley & Sons, New York (1967). Such organic materials, when subjected to light in the wavelength range of the near u.v., provide photoelectrons and a resulting electrical current. (See also Electronic Processes in Organic Crystals, ed. M. Pope and C. Swenberg, Clarendon Press, Oxford, 1982.)
The photovoltaic effect has been shown useful in a variety of processes, such as the generation of a photovoltage in solar cells in photoinduced chemical conversion. Similarly, the photoconductive effect has been used to generate a current that, in turn, is applied to resistance control devices such as photodetectors. The use of organic materials in these applications is considered alluring because of versatile properties and fabrication ease. Thus, research has been directed to the discovery of organic and other carbon-based materials that exhibit photoelectronic behavior, and the discovery of each such material has been considered significant.